Mass Number | Definition, Use, Properties, Examples

Definition of Mass Number

The mass number (symbol A, from the German word Atomgewicht [atomic weight]), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus.

what is mass number

It is approximately equal to the atom’s atomic (also known as isotopic) mass expressed in atomic mass units.

The mass number is the sum of the numbers of protons and neutrons in an atom. It is a whole number.

Example of Mass Number

example of mass number

Cl has a mass number of 37. Its nucleus contains 17 protons and 20 neutrons.

The mass number of carbon-13 is 13. When a number is given following an element name, its isotope states the mass number. To find the number of neutrons in an isotope atom, subtract the number of protons (atomic number). So, carbon-13 has 7 neutrons because Carbon has atomic number 6.


The atomic mass of a single atom is its total mass and is typically expressed in atomic mass units or amu. By definition, an atom of Carbon with six neutrons, carbon-12, has an atomic mass of 12 amu. Other atoms don’t generally have round-number atomic masses. In general, though, an atom’s atomic mass will be very close to its mass number but will have some deviation in the decimal places.         

Use of Mass Number

use of mass number

The mass number helps to give an idea of the isotopic mass. Isotopic mass is measured in atomic mass units or “u.”

An isotope of an element will have the same atomic number but a different mass number. Isotope mainly differs in the number of neutrons. Different isotopes of the same element will have a different mass numbers.

However, isotopes of different elements can have the same mass number, such as carbon-14 (6 protons + 8 neutrons) and nitrogen-14 (7 protons + 7 neutrons).

Characteristics of Mass Number

Characteristics of Mass Number

The mass numbers is an integer (whole numbers) equal to the sum of the numbers of protons and neutrons of an atomic nucleus. In other words, it is the sum of the number of nucleons in an atom.

The mass number is often denoted using a capital letter A. Contrast this with the atomic number, which is simply the number of protons.

Properties of Mass Numbers

Properties of Mass Number

The various properties of mass numbers are enumerated here:

1. Sum of protons and neutrons provide this number of a certain element.

2. It is represented by the letter A.

3. Protons and Neutrons are together termed as nucleons.

4. Example: Atoms of a carbon consist of 6 protons and 6 neutrons. Therefore, the mass number of Carbon is 12.

5. The number of neutrons may vary in an element. However, the total number of protons is the same in all atoms of an element. Therefore, the atoms of the same element with the same atomic numbers but a different mass numbers are termed isotopes.

6. Generally, atomic mass and mass numbers are two different terms and may vary slightly. In most cases, they are not the same. However, the weight of an electron is almost negligible, so we can consider the atomic mass of an atom to be almost equal to its mass numbers.

How to Find Atomic Mass for a Single Atom

How to Find Atomic Mass

Since the combined masses of protons and neutrons account for almost all the mass of the given atom, the atomic mass of the single atom can be calculated by adding the total numbers of protons and the total numbers of neutrons of that particular isotope.

The number of protons in a given atom is always equal to its atomic number. For example, the atomic number of oxygen is 8. Therefore the total number of protons in an oxygen atom is 8. The total number of neutrons is generally specified when describing which isotope the atom belongs to.

Example: Calculation of the atomic mass of an oxygen molecule with 9 neutrons.

Total number of protons in oxygen = Atomic number of oxygen = 8

Total number of neutrons in the isotope (given) = 9

Atomic mass of the given oxygen atom = 8+9 = 17


The only time the atomic numbers and mass numbers are the same is when you deal with the protium isotope of hydrogen, which consists of a single proton. When considering elements in general, remember the atomic numbers never changes, but the mass numbers may change because there may be multiple isotopes. 

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